This invention relates to gas filtration apparatus and process, and more particularly to gas filtration apparatus and process which combines sonic agglomeration with porous cross flow filtration.
Gas streams comprising small particles are produced by many chemical processes and by combustion processes, such as the particulates in the effluent gas stream of coal gasifiers. Such particulate matter and frequently, undesired gases should be removed before the gas stream is passed to downstream equipment or processes or is released to the atmosphere. It has been particularly difficult to remove such particulate matter from high temperature and high pressure gas streams frequently encountered. Many of the coventional methods, such as cyclones and electrostatic precipitation fail under high pressure conditions. Particles which are over 10 microns in diameter may be removed from gas streams by conventional porous filters and the like, but smaller particles are difficult to remove because the porosity of the filter must be so small that it creates a substantial pressure drop across the filter. This is undesirable because the higher pressure drop requires greater energy and may inhibit the combustion process. Thus, porous flow through filters do not remove small particles from gas streams efficiently.
Particles in a turbulent gas stream collide with each other and may agglomerate with other particles on impact to form larger agglomerates. The number of collisions may be increased by confining the stream in a tube or flue, and subjecting the particles in the flue to a sonic field. This process, known as sonic agglomeration, has been used for many years to agglomerate small particles into larger agglomerates.
Sonic agglomerators have been used in combination with inertial separators, such as cyclones, as described in U.S. Pat. Nos. 2,935,375 and 3,172,744. Cyclones spin the gas stream, exerting centrifugal force on the particles in the stream. The centrifugal force propels the particles against the outer wall of the cyclone, from which they may be reoved. The agglomerates may break up into many small particles on impact with the outside wall, however, which negates the beneficial effects of the sonic agglomerator. Also, small particles may follow the air stream through the clean gas exit rather than be removed by the device. U.S. Pat. No. 3,681,009 teaches introducing secondary particles of water vapor into a particulate contaminated gas stream and then exposing the gas stream to a sonic field causing agglomeration followed by removal of the agglomerates from the gas stream in a cyclone separator.
Non-inertial capture systems, such as electrostatic precipitation, do not suffer these limitations of cyclones, but are difficult to utilize at elevated temperatures and pressures due to problems such as electric arc breakdown.
Water vapor may be introduced into a gas stream to trap and remove particles which have been subjected to sonic agglomeration, as in U.S. Pat. No. 3,390,869, and water may be passed over a screen filter to remove agglomerated particles, as in U.S. Pat. No. 3,763,634. These systems are impractical in applications in which the gas stream is at temperatures, for instance, between about 200.degree. C. and 1000.degree. C., because the water vaporizes. U.S. Pat. No. 3,834,123 teaches agglomeration of dust particles by ultrasonics, which is stated to be unreliable and requires too high an expenditure of energy for use in combination with pocket or bag textile filters and suggests recycling dust released from the filter to the contaminated gas stream. This patent teaches the necessary reverse flow cleaning cycle when using a textile filter. U.S. Pat. No. 2,769,506 teaches vibration of bag filters by sound waves to free collected aerosols from the external surfaces of the bags. Bag filters, of course, cannot be used at high temperatures.
None of the prior art known to the applicants suggests the combination of sonic agglomeration followed by cross flow filtration which provides removal of small particles, under 10 microns diameter, on a continuous basis from a high temperature gas stream.
It is an object of this invention to provide apparatus and process for removal of micron and sub-micron particles from a gas stream utilizing the combination of sonic agglomeration and cross flow filtration.
It is another object of this invention to provide apparatus and process for removal of particulates from gas streams at temperatures of about 200.degree. C. to about 1000.degree. C.
It is yet another object of this invention to provide apparatus and process for continuous removal of particulates from gas streams avoiding the need for reverse flow cycles for cleaning filters.
It is still another object of this invention to provide apparatus and process for continuous removal of particulates from high temperature gas streams while incurring a relatively small pressure drop across the apparatus.